1. Field of the Invention
The present invention relates to an electrochemical method for determining the end point of organic or inorganic reactions and an electrode used in the method. More particularly, the invention pertains to a method for electrochemically detecting the end point or monitoring the status of an organic reaction, and in particular diazotization, or azo coupling reactions or other organic or inorganic reactions, in which an oxidation-reduction reaction, an exchange reaction or a precipitation reaction takes place, by utilizing potentiometric or amperometric techniques, and to a gold electrode effective to carry out the method.
2. Description of the Prior Art
It is quite important to precisely detect the end point of an oxidation-reduction reaction of organic compounds or inorganic compounds or the like, in order to improve the quality of the products resulting therefrom, and to make the reaction more economical.
For instance, among dyestuffs, azo dyestuffs having azo groups (--N.dbd.N--) as the chromophore may be used as the starting material for deriving various kinds of other dyestuffs by combining them with other chromophores or auxochromes. The azo dyestuff makes it possible to obtain dyestuffs belonging to acid dyestuffs, direct dyestuffs, basic dyestuffs, mordant dyestuffs, insoluble azo dyestuffs and pigments. In light of their dyeing ability and the color tone of these dyestuffs, derived from azo dyestuff, almost their whole range is extended. At least a third of the dyestuffs presently on the market fall within this kind of dyestuff, since these can relatively economically and easily be prepared by combining diazotization and azo coupling reactions.
In the diazo coupling reaction, hydrochloric acid is generally used and, in particular, it should be used in a large amount if the reaction is carried out under the condition such that a diazoamino compound may easily be formed in the reaction system. Moreover, the diazonium salt formed during the diazotization reaction is relatively unstable and therefore, the subsequent coupling reaction may be impaired by the excess nitrous acid which may still be present after the diazotization reaction. In such a case, one must pay careful attention to the reaction so as to check the presence of nitrous acid and the nitrous acid possibly present must be decomposed by sulfamic acid (H.sub.2 NSO.sub.3 H) or urea (H.sub.2 NCONH.sub.2) prior to subsequent reactions. If such careful attention is not paid, the quality of the product becomes quite low. In order to eliminate the detrimental effects mentioned above, therefore, it is necessary to correctly detect the end point of the reaction concerned, exactly estimate the amount of excess nitrous acid at that time and eliminate it before recovering and storing the resulting product. Therefore, there is a great need for developing a technique for detecting an exact end point and estimating an exact amount of residual nitrous acid present in the reaction system.
It is also important to monitor the status of reactions at each desired stage during the reaction. For example, in the case of diazo dyestuff synthesis, it is known that the secondary coupling reaction is started immediately after the completion of the primary coupling reaction. However, if one wants to recover the product of the primary coupling reaction, it is necessary to find conditions therefor by monitoring the reaction process exactly. In addition, monitoring of the reaction process is effective to estimate and control the rate of reaction.
Conventionally, as the method for monitoring reaction processes and detecting the end point of a diazotization reaction or an azo coupling reaction, there is known, for example, a method which comprises detecting the presence of excess nitrous acid by examining the coloration of potassium iodide-starch paper as the external indicator (see, for example, JIS K4101 11, 3-3, 1980), in the case of diazotization, and a method which comprises effecting color reaction on a filter paper and estimating the extent of reaction from the color developed in the case of an azo coupling reaction (see JIS K4101 11, 2-1).
However, in these conventional methods, an operator must judge the extent of the reaction or the end point of the reaction according to his own visual color judgment and thus, the judgment would not be a complete one. In addition, such visual estimation requires well experienced personnel.
Under these circumstances, there has been proposed an electrochemical method for monitoring reaction processes and determining the end point thereof, and, as a typical technique there may be mentioned a method such as a polarographic method. The polarographic method comprises, for example, applying a potential, which linearly increases with time, to a sample to be examined and analyzing the observed electrical current variation with time. In the method, a dropping mercury electrode and a counter electrode of mercury pool having a large electrode surface are generally used for an aqueous solution system.
As the method for detecting the end point and monitoring processes of diazotization or diazo coupling reactions according to the polarography technique, a method comprising observing and analyzing an electrical current variation due to the electrolysis of a diazo or azo compound formed during the reactions (see, Japanese Patent Publication No. 28719/73); a method using electrodes in which the sensing portion thereof is composed of platinum (JIS K4101 11, 2-1) and a method using a pair of electrodes, each of which has the sensing portion composed of a member selected from the group consisting of graphite, carbon, platinum or the like have been reported (see, for example, "Analysis method of K acid" issued by Bayer A. G., 1980).
However, there are many problems encountered in carrying out these conventional methods for electrochemically detecting the end point and processes of the reaction concerned, such that an apparatus provided with an electrolytic mechanism such as a dropping mercury electrode which is essential in the polarography technique, is necessary. And, if the electrochemical property of an amino group, a phenolic hydroxyl, an active methyline or a diazo group which takes part in the objective reaction is quite low, and an electrode of graphite, carbon, or platinum or a combination thereof is adopted, it is often observed that there is no detectable response such as a variation of the electrical potential or current. In particular, the reduction in sensitivity due to the contamination of the electrode surface makes the accuracy of the measurement quite low and constitutes an inherent disadvantage of the electrochemical method and apparatus, when monitoring the processes of a series of reactions without replacing the electrodes used.
Furthermore, in determining the end point of a coupling reaction, the following spot test is generally adopted, which comprises collecting a sample during the coupling reaction, dropping the sample on a filter paper to form a colorless moistured area, contacting a diazo solution with a part of the colorless area and an H-acid solution (indicator) with the other part of the colorless area (JIS K4101 11, 3-3, 1980). However, the spot test is believed to be effective only to determine the end point of a monoazo coupling reaction and is useless in determining the end point of the reaction in which at least two coupling reactions successively take place such as diazotization and trisazotization reactions. In such a case, the end point is indirectly determined by detecting the unreacted monoazo component according to a liquid chromatography technique or the like.
In the light of the aforementioned situation of the method for detecting the end point and monitoring the processes of an organic or inorganic reaction, it is industrially quite important to develop a new technique capable of solving the problems encountered in the conventional method in order to accurately detect the end point, to make the electrochemical analysis of various kinds of reactions and the process control thereof easier and to obtain an objective product more economically and effectively.